The present invention generally relates to semiconductor devices, and more particularly to a semiconductor device having a circuit construction capable of preventing destruction of the device due to static electricity, and suitable for applicaton to bipolar integrated circuits.
Generally, accompanied by the developments in the Large Scale Integrated (LSI) circuits, bipolar elements for constructing a bipolar integrated circuit are further miniaturized year by year. However, as the bipolar elements are further miniaturized, the elements are more easily destroyed by static electricity which is inevitably introduced upon handling of the elements. Accordingly, measures to prevent the electrostatic destruction are being stressed.
Thus, conventionally, as a measure to prevent the electrostatic destruction, a circuit construction has been used in which the cathode of a diode is connected between the input pad of the integrated circuit and the input transistor, for example, and the anode of the diode is connected to the power source. The input transistor of a CML or ECL circuit which requires high performances, is generally an NPN-transistor. When a positive electrostatic pulse is applied to the input pad, the input transistor is not destroyed by the electrostatic pulse since the electrostatic pulse is in the forward direction with respect to the base-collector junction of the input transistor. However, when a negative electrostatic pulse is applied to the input pad, the input transistor is reverse-biased, and when the above diode is not provided, the junction of the input transistor is destroyed in a case where the electrostatic pulse is large. In a case where the above diode is connected, however, the diode becomes active when a negative electrostatic pulse is applied to the input pad, and the large instantaneous current which flows at the initial stage when the negative electrostatic pulse is applied, flows toward the input pad through the diode. Hence, the distribution of the input transistor is prevented.
However, as the performance of the integrated circuit itself becomes higher and higher, the following notable disadvantages are introduced by use of the conventional circuit described above. That is, a large voltage is generated between the terminals of the diode due to an internal resistance of the diode when the large instantaneous current flows at the initial stage of applying the electrostatic pulse, and a desirable destruction preventing effect cannot be expected since the input transistor could possibly be destroyed. Therefore, a diode having a large surface area must be used in order to reduce the internal resistance of the diode. But on the other hand, this type of a diode having a large surface area comprises a large junction capacitance. Accordingly, when this type of a diode is used, although the internal resistance of the diode is small, the input signal waveform is distorted due to the large junction capacitance of the diode, and is disadvantageous in that the high-speed operation of the integrated circuit is disturbed.
Hence, accompanied by the upgrading in the high performance of the integrated circuits, the realization of a measure for preventing electrostatic destruction of a device which does not use the above diode, is highly desired.